Carbon dot light emitting diodes

ABSTRACT

An electroluminescent LED device comprising a hole transport layer, an electron transport layer, an active emissive layer between the hole transport layer and the electron transport layer, and carbon dots forming the active emissive layer.

CROSS REFERENCE TO RELATED APPLICATIONS/PRIORITY

The present invention is a divisional of U.S. patent application Ser.No. 14/940,336 filed Nov. 13, 2015, which claims the benefit of U.S.Provisional Application No. 62/079,612 filed Nov. 14, 2014, which isincorporated by reference into the present disclosure as if fullyrestated herein.

FIELD OF THE INVENTION

The present invention relates to the field of color display andwhite-light lighting. The invention comprises light-emitting diodes(LEDs) for color display and white-light lighting.

BACKGROUND OF THE INVENTION

Light-emitting diodes (LEDs) offer great prospects for developinglow-cost, efficient, bright, and large area color displays andwhite-light lighting. Traditional LEDs use rare earth or organicluminescent materials. Quantum dots (QDs) based LEDs (QD-LEDs) exhibitsize tunable spectral emission, allowing for the design and fabricationof color QD-LEDs with simple device configurations for individual pixelbased color elements. However, a serious drawback of the presentcolloidal QD-LED technology is its dependence on the QDs with toxicheavy-metal components, such as cadmium, lead, and mercury which aredetrimental to the environment and therefore potentially hinder thecommercialization. Carbon nanoparticles (also called carbon dots,abbreviated as CDs) are recently developed new materials. They are easyto make from simple raw materials. Carbon is the earth abundantmaterial, safe, biocompatible, and cheap. CDs also show different colorsunder light excitation (photoluminescence) with very high quantumyields. The inventors developed CD-based LEDs (CD-LEDs) for colordisplay (e.g., computer monitors, TVs, phone screens, entertainmentlighting, instrument back lighting) and white-light solid-state lighting(for buildings, rooms, personal reading/study, public areas, roads,etc.).

SUMMARY OF THE INVENTION

Wherefore, it is an object of the present invention to overcome theabove mentioned shortcomings and drawbacks associated with the priorart.

The present invention uses carbon nanoparticles that can be nanosizedcrystalline or amorphous carbon particles, small graphene, short carbonnanotubes (single or multiple walls), with confining size varying from,preferably 0.5 nm to 20 nm, and more preferably 2 nm to 10 nm, as theelectroluminescence materials to make light-emitting diodes (LEDs) forcolor display and white-light lighting. This invention adjusts injectioncurrent density by changing the thicknesses of the carrier transportlayers and/or the electrode materials for different color andwhite-light emission.

The present invention also relates to an electroluminescent LED devicecomprising a hole transport layer, an electron transport layer, anactive emissive layer between the hole transport layer and the electrontransport layer, and carbon dots forming the active emissive layer.

The present invention also relates to a method of producing lightcomprising the step of supplying a current of electricity to anelectroluminescent LED device, wherein the electroluminescent LED devicecomprises a hole transport layer, an electron transport layer, an activeemissive layer between the hole transport layer and the electrontransport layer, and carbon dots forming the active emissive layer.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate various embodiments of theinvention and together with the general description of the inventiongiven above and the detailed description of the drawings given below,serve to explain the principles of the invention. It is to beappreciated that the accompanying drawings are not necessarily to scalesince the emphasis is instead placed on illustrating the principles ofthe invention. The invention will now be described, by way of example,with reference to the accompanying drawings in which:

FIG. 1 is a schematic view, showing the general structure of theCD-LEDs;

FIG. 2 is a transmission electron microscopy (TEM) image of CDs, with aninset showing high resolution transmission electron microscopy (HRTEM)image of a single dot;

FIG. 3 shows the ultraviolet-visible (UV-Vis) absorption andphotoluminescence (PL) (340 nm excitation wavelength) spectra of a CDthin film spin-coated on quartz glass;

FIG. 4 shows a true-color photograph of CD emission (excited by 340 nmlight) in solution;

FIG. 5 shows current density and brightness of the CD-LEDs with an insetshowing the device structure comprising ITO/PEDOT:PSS (anode), poly-TPD(hole transport layer), CDs (active emissive layer), TPBI (electrontransport layer), and LiF/Al (cathode);

FIG. 6 is the luminous and power efficiencies vs. current density forthe same CD-LEDs in FIG. 5;

FIG. 7 shows the electroluminescence (EL) spectra and the true colorphotographs of blue, cyan, magenta, and white emissions for the sameCD-LEDs in FIG. 5;

FIG. 8 gives the Commission Internationale de l'Enclairage (CIE) 1931coordinates of the blue, cyan, magenta, and white emission from the sameCD-LEDs in FIG. 5 operated at different voltages;

FIG. 9 shows the PL spectra of CDs excited by 3 different excitationwavelengths; the color-coded arrows represent the detection wavelengthfor the PL decay curves shown in FIG. 10;

FIG. 10 gives the PL decay curves of CDs under 320 nm excitationdetected at different wavelengths (420-580 nm);

FIG. 11 shows the current density and brightness of the CD-LEDs with adifferent structure (using a 5 nm LiF layer) which is depicted as theinset;

FIG. 12 is the luminous and power efficiency vs. current density for theCD-LEDs in FIG. 11;

FIG. 13 shows the EL spectra and images of the same CD-LEDs in FIG. 11operated at voltages of 5, 7, and 9V, respectively; These CD-LEDs emitblue light only; see the true color photographs.

FIG. 14 shows the current density and brightness of a third structuredCD-LEDs using ZnO nanoparticles as an electron transport layer;

FIG. 15 is the luminous and power efficiency vs. current density for theCD-LEDs in FIG. 14; and

FIG. 16 shows the EL spectra of the CD-LEDs, with true-color photographsof the LEDs operated at different applied voltages. These CD-LEDS emitwhite light only.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be understood by reference to the followingdetailed description, which should be read in conjunction with theappended drawings. It is to be appreciated that the following detaileddescription of various embodiments is by way of example only and is notmeant to limit, in any way, the scope of the present invention.

Turning now to FIG. 1, a brief description concerning the variouscomponents of the present invention will now be briefly discussed. Thecarbon dots employed in the CD-LED devices can be synthesized by manyexisting methods. One approach is based on the publication of Wang, F.;Pang, S.; Wang, L.; Li, Q.; Kreiter, M.; Liu, C.-Y. One-Step Synthesisof Highly Luminescent Carbon Dots in Noncoordinating Solvents. Chem.Mater. 2010, 22, 4528-4530 using 1-octadecene as the noncoordinatingsolvent, 1-hexadecylamine as the surface passivation agent, andanhydrous citric acid as the carbon precursor. Such process incorporatedby reference. The CDs are purified from the synthesis solution through apreferably multi-step precipitation/re-disperse process and subsequentlydried as solid powders.

According to one embodiment, ZnO nanoparticle is used for thewhite-light LEDs. To synthesize ZnO nanoparticles, a mixture of 0.44 gzinc acetate and 30 mL ethanol is loaded into a three-neck flask andheated to 75° C. until a clear solution is obtained. After the solutionis cooled down to room temperature, 10 ML NaOH/ethanol solution (0.5mol/L) is injected into the flask. The solution is stirred for 12 hours,and the products are collected by precipitating with hexanes andre-dispersed in ethanol for device fabrication.

The general CD-LED device fabrication, shown in FIG. 1 starts with aUV-ozone treatment of a transparent conducting film as the anode 1. Thistransparent conducting film can be indium tin oxide (ITO), fluorinedoped tin oxide (FTO), carbon nanotube networks, or graphene. Thetreatment is to enrich the surface with oxygen and, consequently,increase the anode film's work function. A hole injection layer (HIL) 2of poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) (10to 100 nm) is then deposited on the UV-ozone treated anode 1 byspin-coating, followed by annealing in an oven at 120° C. for 10 min inair. Then, the sample is transferred into a nitrogen glove box system;the hole transport layer (HTL) 3 ofpoly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl) benzidine) (poly-TPD) orother materials like poly(N-vinylcarbazole) (PVK), 10 to 100 nm inthickness, is spin cast on the top of the HIL 2 from their chlorobenzenesolution and cured at 150° C. for 30 min on a hot plate. Subsequently,the CD-active emissive layer 4 (10 to 100 nm) is made by spin-coatingover the surface of HTL 3 from its toluene solution and is baked on ahot plate at 80° C. for 30 min to form the active region of the CD-LED.The thickness of the CD-active emissive layer 4 is precisely tailored,by varying the CD concentration and the spin speed of the castdeposition, to balance the maximum brightness and emission efficiency ofthe CD-LEDs. Next is to make the electron transport layer (ETL) 5. Forthe 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI)-based device, a2 to 50 nm thick-TPBI ETL is thermally deposited over the CD-activeemissive layer 4, followed by a LiF/Al (1 to 20 nm/10 to 300 nm) bilayercathode 6 which is thermally evaporated through a shadow mask withoutbreaking the vacuum. For the ZnO nanoparticle-based device, a 5 to 100nm thick ZnO nanoparticle ETL is deposited over the CD-active emissivelayer 4 by spin coating, and an Al (10 to 300 nm) cathode 6 is thermallyevaporated through a shadow mask (no LiF layer).

The photoluminescence (PL, light emission from materials after theabsorption of photons) spectra of CDs are recorded by PLphotospectroscopy and time-resolved, pulsed laser excitation techniquesare used for probing the lifetime of PL emission; these measurements andanalyses can be used for the electron-hole recombination mechanism ofthe electroluminescence (EL, a material emits light in response to thepassage of an electric current or to an electric field). The electricalcharacterization of the devices is performed on a Keithley 2400 sourcemeter. The EL spectra and luminance of the devices (cd/m²) are measuredon a PR650 spectrometer. Images of the LED outputs are recorded with aSony FWX700 FireWire color CCD camera. All measurements are performedunder dark condition.

The light color from the CD-LEDs is current density-dependent for thesame CD particles. FIG. 2 shows the transmission electron microscopy(TEM) image of the carbon dots with an average diameter of 3.3 nm (onecrystalline spherical CD particle of 1 nm in diameter contains about 30to 80 carbon atoms); inset is a high resolution TEM image to show thecrystalline structure of the CDs. Shape and size difference, hollow orsolid particles, crystalline or amorphous structures, graphene or shortcarbon nanotubes (single or multiple walled) affect the actualwavelength of the emitting light, the efficiency, the luminance of theLED devices, and the color properties of the light (such as colortemperature, color rendering index, color purity, CIE coordinate) andthe device parameters (such as the thickness and materials of each layerin the device). The absorption spectrum of a CD film and the PL spectrumof the CD solution in toluene at an excitation wavelength of 340 nm arepresented in FIG. 3, which exhibit three absorption peaks at 270, 315,and 450 nm, and a main PL peak at 420 nm respectively. FIG. 4 shows thetrue-color photograph of this emission. The full width at half maximum(FWHM) bandwidth of the luminescence band is between 90 and 95 nm, whilethe PL QY is determined to be ˜40%, by using a spectrometer with anintegrating sphere and a back-thinned CCD detector. The above mentionedcharacteristic data may vary with different type, size, and surfacepassivation of the CDs.

A schematic of the blue-white CD-LED device structure used in thisinvention is shown in the inset of FIG. 5. The device consists of apatterned ITO anode 1, a 25-nm thick PEDOT:PSS HIL 2, a 40-nm poly-TPDHTL 3, a 20-nm CD-active emissive layer 4, a 5-nm ETL 5, and a 1-nm LiFand 150-nm aluminum double layer as the cathode 6. PEDOT:PSS is used asa buffer layer on the anode mainly to increase the anode work functionfrom 4.7 (ITO) to 5.0 eV and to reduce the surface roughness of theanode to obtain stable and pin-hole-free electrical conduction acrossthe device. Poly-TPD is used as the HTL in consideration of the factthat its highest occupied molecular orbital (HOMO) level is 5.2 eV whichis very close to the work function of the ITO/PEDOT:PSS, and alsobecause it possesses an excellent hole-transport capability. Moreover,poly-TPD has been found to have good solubility in organic solvents suchas chlorobenzene and thus makes it easy to form a uniform thin layer.TPBI is chosen as the ETL because of its good electron-transportcapability and its interfacial phase compatibility with the CD layer.

FIG. 5 shows the typical current density and luminance curves as afunction of applied voltage for the CD-LEDs. The variation of the ELefficiency, across the entire measured luminance range and bias, isshown in FIG. 6. The devices demonstrate a low turn-on voltage of 5 V,confirming the reduced barrier height for charge injection into theCD-LEDs. The highest luminance and luminous efficiency reach 61 cd/m²and 0.018 cd/A, respectively. The EL spectra of the CD-LEDs, operated atdifferent voltages, are shown in FIG. 7; images of the CD-LEDs underoperation are presented in the insets of the figure. The CD-LEDsexhibited bright, uniform and defect-free EL emission. There is noobvious contribution from the polymer HTL or organic ETL in the ELspectra while the similar EL spectra can be obtained from the LED devicewithout the polymer HTL or organic ETL. The CIE coordinates of theemitted lights of the CD-LEDs are (0.198, 0.151), (0.212, 0.162),(0.260, 0.221), and (0.318, 0.320) as shown in FIG. 8, corresponding toblue, cyan, magenta, and white lights, respectively. Thecurrent-voltage-luminescence and EL spectra can be achieved repeatedlyfor sealed CD-LEDs.

The light color from the CD-LEDs is apparently voltage-dependent for thesame 3.3 nm CD particles (FIG. 7). The blue emission peak at 426 nm inFIG. 7 can be observed at a low bias of 6 V. With the increase of bias,the emission peak at 452 nm becomes stronger, and the color becomes cyanat the bias of 7 V. With the further increase of bias, the emission at588 nm appears and becomes stronger changing the emission hue tomagenta, and finally at higher driving voltage gives white emission fromthe CD-LEDs. It can be observed that there are three main emissions,peaked at 426, 452 and 588 nm in the white EL spectra, appearssuccessively with increased bias (FIG. 7). Thus, the white emission athigh bias is probably due to the appearance of multiple-recombinationprocesses at high current density. This phenomenon is probably a resultof the fact that several recombination mechanisms with different but nottoo dissimilar (final) excited state lifetimes co-exist in each singleCD, allowing several recombination pathways without any particular onedominating completely.

The excitation-dependent PL spectra and PL decay curves are recorded inorder to understand the recombination processes in the CDs and hence tofurther control the device emission. FIG. 9 shows the evolution of thePL emission spectra of the CDs, which exhibits excitation-dependentemission ranging from 400 to 700 nm, similar to the literature reports.When the CDs are excited by a short-wavelength light of 340 nm, a blueemission peaked at 420 nm is observed. When the excitation wavelength is400 nm, the emission peak shifts to 460 nm, the emission at 420 nmbecomes relatively weaker whilst a small emission peak at 580 nmappears. With a longer exciting wavelength of 480 nm, the emission peakat 580 nm becomes stronger. These three peaks are consistent to thoseobserved in the EL spectra of CD-LEDs.

The absorption spectrum of the CD films exhibits three peaks at 270,315, and 450 nm as shown in FIG. 3. The peak at 270 nm can be ascribedto a π-π* transition of aromatic C═C bonds, while the peak at 315 nm maybe attributed to an n-π* transition of C═O bonds and the broad peak at450 nm may originate from the amino-functionalized surface of the CDs.After the excitation wavelength changes from 340 to 400 nm, the strongemission peaked at 420 nm disappears probably because the excitationenergy is not sufficient to drive the π-π* transition. When theexcitation wavelength changes to 480 nm, only the amino group-relatedemission peaked at 588 nm can be observed. That means that each of thethree emission processes of the CDs relies on the excitation and can beselectively controlled via the absorption energy. It has to be notedthat this phenomenon is quite similar to the recent reported PL fromgraphene dots, which also exhibits such selectable property attributedto independent molecule-like states through femtosecond transientabsorption spectroscopy and femtosecond time-resolved fluorescencedynamics investigation.

PL decay curves have been analyzed in order to further characterize theorigin of the emission components of the PL in CDs (FIG. 10). Theradiative lifetime of PL emission is an important characteristic oflight-emitting nanoparticles. Different radiative lifetimes maycorrespond to different electron-hole recombination mechanisms.Time-resolved, pulsed laser excitation techniques are most suitable forprobing the lifetime of PL emission. FIG. 10 shows the representative PLdecay curves of the CDs, which are probed at different emissionwavelengths. The decay fitting results are listed in Table 1, withradiative lifetime τ₁ of 2 ns, τ₂ of 5˜6 ns, and τ₃ of 14˜15 ns. It isfound that the PL decays of the CDs are emission wavelength dependent.The amplitudes A₁ and A₂ with radiative lifetime of τ₁ and τ₂ accountfor a large amount of the PL emission spectra at short wavelength. Theproportion of amplitude A₁ decreases and the strength of the processesassociated with A₃ increases as the emission wavelength shifts from 420to 580 nm while A₂ does not vary that much. Therefore, the shortlifetime of 2 ns can be attributed to the emission peaked at 420 nm, themedium one of 5˜6 ns corresponds to the emission peaked at 460 nm, andthe long lifetime of 14˜15 ns can be attributed to the emission peakedat 580 nm. Normally, relaxations (prior to radiative relaxation)involving the emission of high energy phonons will be more rapid (and soare associated with the fastest decay energy level). Therefore, when theexcitation energy is high (e.g., 340 nm), the emission peaked at 420 nmis the dominant recombination channel as shown in FIG. 5.

TABLE 1 Fitted decay times and normalized amplitudes of the PL emissionof CD film at different wavelengths under 320 nm excitation Emissionwavelength/nm τ₁/ns A₁/% τ₂/ns A₂/% τ₃/ns A₃/% 580 2.1 19.1 5.5 47.614.0 33.3 540 2.1 19.6 5.8 49.4 15.0 31.0 500 2.0 21.1 5.8 49.9 15.129.0 460 2.0 28.4 5.4 49.5 14.8 22.1 420 2.0 32.4 5.1 48.3 14.0 19.3

Correspondingly, for the CD-LED's EL emission, when the injectioncurrent density is low, the carriers preferentially relax via the energylevel associated with the faster decay channel. Thus blue emission isseen at low current density. When the current density is high enough,other emission colors (associated with slower recombination rates) areobserved in the CD-LEDs. Therefore, the CD-LED's color-switchable EL isactually driven by the injection current density, not the apparentlyapplied voltage.

According to the fluorescence lifetimes of different emission states ofthe CDs, the steady state PL spectra (FIG. 9) indicate that theseemissive states are distinct energy levels (centered on 420, 460 and 580nm) that have their own distinct excitation spectra. In the case ofelectroluminescence, electrons and holes are injected from the chargeinjection layers into the active emissive layer and only specific typeof states will he excited. Since excitation of the specific states inthe CDs are distinct (420, 480 and 580 nm) as shown in FIG. 9, theexcited state that is formed via injected charges would emanate from thecorresponding emission level associated with each color. The energystate with short lifetime (420 nm) will be very readily depopulated.Therefore, at low current density (or voltage), the carriers willinitially be injected into the energy state with short lifetime due tothis fast relaxation. When the current density is increased, the morehighly populated high energy state can also feed the low energy states(i.e., 480 and 580 nm) and so emission from these levels becomes moresignificant alongside the direct relaxation from the 420 nm state.

EXAMPLES

Blue emitting CD-LED device. Taking these findings into consideration,the CD-LED device structure is adjusted to control the current densityand therefore the EL spectra. One way is to reduce the injection currentdensity by increasing the thickness of the LiF layer to 5 nm (the insetof FIG. 11). FIG. 11 shows typical current and luminance curves as afunction of the applied voltage for the pure blue emitting CD-LEDs witha cathode of 5-nm LiF and 150-nm Al. The devices exhibit a low turn-onvoltage of 5 V, confirming a similar minimized barrier for chargeinjection into the CD-LEDs. The maximum luminance obtained is 24 cd/m².The current density is reduced to ˜150 mA/cm² due to the increasedthickness (correspondingly the increased resistance) of the LiF layer (5nm). The variations of the power efficiency and the luminous vs. currentdensity are shown in FIG. 12. The EL spectra of the blue CD-LEDs,operating at different voltages, are shown in FIG. 13; emitting imagesof the CD-LEDs under operation are also presented in the figure insets.The blue-emitting CD-LED exhibited bright, uniform and defect-free ELemission over the whole bias working range when the current injectionwas maintained low.

White emitting CD-LED device. Low current injection can be used toobtain pure blue emission from the above CD-LEDs. The inventorstherefore deduce that high current injection would lead to whiteemission in the whole working bias. To realize it the inventors use ZnOnanoparticles as the ETL. The ZnO/CD-LED structure shown schematicallyin the inset of FIG. 14, consists of layers of ITO/PEDOT:PSS (25 nm,anode), poly-TPD (40 nm, HTL), CDs (20 nm, active emission layer), ZnOnanoparticles (10 nm, ETL), and Al (150 nm, cathode). FIG. 14 also givesthe current density and luminance changes of the ZnO/CD-LEDs. Thevariations of the power efficiency and the luminous vs. current densityare shown in FIG. 15. The maximum luminance for the resulting whitelight emitting devices reaches 90 cd/m². These ZnO/CD-LEDs exhibitturn-on voltages of 4.6 V, which is lower than those of the TPBI-baseddevices. The EL spectra measured using a high-sensitivity spectrometerindicate that the light emission from the white CD-LEDs is achieved at adriving voltage as low as 4.6 V, suggesting that electrons and holes canefficiently inject into the CD emitting layer at lower driving voltages.These devices have significantly higher current density than the deviceswithout an ETL, or with a TPBI ETL. As the same HTLs are used in all theCD-LEDs, the high current density is attributed to the more efficientelectron injection into the CD layer with a ZnO ETL. This is due to thehigher electron mobility of the ZnO nanoparticles which has beenreported to be 2×10⁻³ cm² V⁻¹ s⁻¹, at least one order of magnitudehigher than that of organic ETLs (typically 1×10⁻⁴ cm² V⁻¹ s⁻¹ orlower). With more electrons accumulated at the poly-TPD/CD interface,the interfacial recombination rate is much higher in the ZnO/CD-LEDsthan that in the other structures. The white light-emitting CD-LEDsexhibit bright, uniform and defect-free EL emission over the whole biasworking range (FIG. 16).

While various embodiments of the present invention have been describedin detail, it is apparent that various modifications and alterations ofthose embodiments will occur to and be readily apparent those skilled inthe art. However, it is to be expressly understood that suchmodifications and alterations are within the scope and spirit of thepresent invention, as set forth in the appended claims. Further, theinvention(s) described herein is capable of other embodiments and ofbeing practiced or of being carried out in various other related ways.In addition, it is to be understood that the phraseology and terminologyused herein is for the purpose of description and should not be regardedas limiting. The use of “including,” “comprising,” or “having” andvariations thereof herein is meant to encompass the items listedthereafter and equivalents thereof as well as additional items whileonly the terms “consisting of” and “consisting only of” are to beconstrued in the limitative sense.

1-16. (canceled)
 17. A method of producing light comprising the step of:supplying a current of electricity to an electroluminescent lightemitting diode (LED); wherein the electroluminescent LED devicecomprises a hole transport layer, an electron transport layer, an activelayer between the hole transport layer and the electron transport layer,and carbon dots form the active layer.
 18. The method of claim 17further comprising the step of varying an injection current densitysupplied to the electroluminescent LED device.
 19. The method of claim17 further comprising the step of changing a color emission by varyingan injection current density supplied to the electroluminescent LEDdevice.
 20. The method of claim 17 wherein the carbon dots are formed ofone of an organic and an inorganic carbon-containing materials.
 21. Themethod of claim 17 wherein the carbon dots are between 0.5 and 20 nm insize.
 22. The method of claim 17 further comprising a transparentconducting film anode, the transparent conducting film including one ofindium tin oxide (ITO), fluorine doped tin oxide (FTO), carbon nanotubenetworks, and graphene.
 23. The method of claim 17 further comprising ahole injection layer (HIL) includingpoly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS), and thehole injection layer having a thickness of between 10 and 100 nm. 24.The method of claim 17 wherein the hole transport layer (HTL) includesone of poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl) benzidine)(poly-TPD) and poly(N-vinylcarbazole) (PVK), the hole transport layerhaving a thickness of between 10 and 100 nm.
 25. The method of claim 17wherein the carbon-dot active emissive layer has a thickness of between10 and 100 nm.
 26. The method of claim 17 wherein the electron transportlayer includes 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI) at athickness of between 2 to 50 nm thick.
 27. The method of claim 26further comprising followed by a LiF/A1 bilayer cathode, whee the LiFlayer has a thickness of between 1 and 20 nm and the A1 layer has athickness of between 10 and 300 nm.
 28. The method of claim 17 whereinthe electron transport layer includes ZnO nanoparticle, where theelectron transport layer has a thickness of between 5 and 100 nm thick.29. The method of claim 28, further comprising an A1 cathode having athickness of 10 between 300 nm.
 30. The method of claim 17 furthercomprising a hole injection layer sandwiched between a transparentconducting film anode and the hole transport layer, and a the electrontransport layer sandwiched between the carbon dot active emissive layerand a cathode.
 31. The method of claim 17 further comprising atransparent conducting film anode, the transparent conducting filmincluding one of indium tin oxide (ITO), fluorine doped tin oxide (FTO),carbon nanotube networks, and graphene; a hole injection layer (HIL)including one of poly(ethylenedioxythiophene):polystyrene sulphonate(PEDOT:PSS), the hole injection layer having a thickness of between 10and 100 nm; the hole transport layer (HTL) includes one ofpoly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine) (poly-TPD) andpoly(N-vinylcarbazole) (PVK), the hole transport layer having athickness of between 10 and 100 nm; and a LiF/Al bilayer cathode, wherethe LiF layer has a thickness of between 1 and 20 nm and the Al layerhas a thickness of between 10 and 300 nm; wherein the carbon dots arebetween 0.5 and 20 nm in size; the carbon-dot active emissive layer hasa thickness of between 10 and 100 nm; the electron transport layerincludes 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) at athickness of between 2 to 50 nm thick; and the electron transport layerincludes ZnO nanoparticle, where the electron transport layer has athickness of between 5 and 100 nm thick.
 32. The method of claim 17wherein the carbon dots are formed of an inorganic carbon-containingmaterial.
 33. The method of claim 17 wherein the light emitting diodedevice is an electroluminescent light emitting diode device.
 34. Themethod of claim 17 wherein the carbon dots are substantially sphericalin shape.